Introduction:

Photosynthetic organism utilizes light energy to create organic compounds from inorganic compounds like water and carbon dioxide. Their skill to package light energy in chemical energy soon supported approximately all other forms of life on planet. Furthermore, the autotrophs radically changed planet and its remaining organisms by decreasing atmospheric concentrations of carbon dioxide, diminishing green house effect, and filling of atmosphere with waste product that some other organisms eventually found necessary for life oxygen. All the oxygen in air which we breathe has been cycled through plants via photosynthesis. All these can only be possible if light is absorbed.

Pigments:

Light striking the object is either reflected, transmitted, or absorbed. Only light this is absorbed have the effect. Light this is absorbed by molecules known as pigment that are colored as they transmit particular colors of light. Black pigments absorb all wavelengths of light, whereas white pigments absorb no wavelength of light.

Pigments in Plants:

Chlorophylls are fat soluble pigments which occur in plants, algae and all but one primitive group of photosynthetic bacteria. Unlike color or hemoglobin that is insignificant for molecule's function as oxygen carrier, color of chlorophyll is important, chlorophyll absorb maximally at wavelengths of 400-500nm (Violet-blue) and 600-700nm (Orange red).

Synthesis of chlorophyll and of many other pigments in plants is stimulated by light that describes why plants grown in dark, (that is etiolated plants) have no chlorophyll. Likewise, light stimulated synthesis of anthocyanin, another plant pigment, describes why apples are always redder on sunny side of apple three. There are many kinds of chlorophyll, the most significant of which is chlorophyll, primary photosynthetic pigment. Chlorophyll a (C₅₅H₇₂O₅N₄Mg) is grass-green pigment whose structure comprises the atom of magnesium (Mg). It happens in all photosynthetic organisms except photosynthetic bacteria, and absorbs maximally at 430 and 662nm. This absorption spectrum illustrates that chlorophyll strongly absorbs visible light accept green light, it reflects and transmits green light, and thus appears green. This absorption spectrum for chlorophyll closely matches graph illustrating how photosynthesis differs with different wavelength of light.

This so called action spectrum illustrates that pattern of photosynthesis closely follows that of chlorophyll a suggesting that chlorophyll a is primary pigment in photosynthesis. Though, absorptive spectrum of chlorophyll a doesn't perfectly match action spectrum of photosynthesis. Thus, we conclude that other pigments should be involved in photosynthesis. Those pigments are known as accessory pigments.

Accessory Pigments:

Plant also contains other pigments. Those pigments extend range of light helpful for photosynthesis by absorbing photons not absorbed by chlorophyll a.

Most common of the accessory pigments in plants are chlorophyll b and carotenoids. Chlorophyll b (C₅₅H₇₀O₆N₄Mg): This is bluish-green pigment which absorbs maximally at 453 and 642nm. It happens in all plants, green algae, and some prokaryotes. Plants generally have about half as much chlorophyll b as chlorophyll a.

Carotenoids: These are accessory pigments which occur in all photosynthetic organisms. They happen in all photosynthetic organisms. They have forty carbons, are fat-soluble and frequently have no oxygen. Carotenoids absorb maximally at wavelength between 460 and 550nm, thus they are red, orange and yellow. Carotenoids are chemically unconnected to chlorophylls and comprise of carbon rings related by long carbon chains comprising alternating single and double bonds. Similar to all accessory pigments like chlorophyll b, carotenoids extend range of photosynthesis by absorbing light which is not absorbed by chlorophyll a. Carotenoids also protect plants against photo-oxidation that occurs when excited chlorophyll transforms oxygen in light energy radicals. The radicals can attract hydrogen from nearby molecules, thus destroying molecules and killing cells. Mutants who lack carotenoid are vulnerable to such radicals that describe why they are bleach-white when grown in light, and soon die. The most common carotenoid is beta-carotene the reddish-yellow pigment comprising of two six-carbon rings linked by eighteen-carbon chain. Beta-carotene absorbs maximally at wavelength between 400 and 500nm. Carotenoids happen throughout plant kingdom and produce colors of avocados, tomatoes, squash, bananas, carrots and some colorful leaves.

Unlike chlorophyll, carotenoids also happen in animals. Animals can't make carotenoids, but they can metabolize and utilize carotenoids from plants. For instance, carotenoids color egg-yolks, insect wings, black ink released by squids, and bodies of some fish and amphibians. Dazzling array of colors of carotenoids comprises green, blue, red, gray, violet, chocolate and black; they result mainly from proteins which are attached to carotenoids. When heated, this protein breaks off and frees carotenoids Oxidizing carotenes generates Xanthophylls that are red and yellow pigments in tomatoes, carrots, leaves, algae (like Fucoxanthin in brown algae), and photo-autotrophic bacteria. Xanthophylls are less proficient at transferring energy in photosynthesis than beta-carotene. Other accessory pigments comprise chlorophylls c and d, phycoerythrin, red pigment and phycocyanin, blue pigment. Phycoeyania and phycoerythrin are accessory pigments in red algae and cyanobacteria. Phycoerythrin is got by exchanging highlighted group. Various forms of phycoerythrin and phycocynin happen in red algae and cyanobacteria.

Chloroplasts:

The green color of leaves is because solar chemical factories known as Chloroplasts, site of photosynthesis in eukaryotes. Other accessory pigments comprise chlorophylls c and d, phycoerythrin, a red pigment and phycocyanin, blue pigment. In plants, photosynthesis happens by in chloroplasts. Light is absorbed and converted to chemical energy in thylaloid and grana; chemical energy is then utilized in stroma to make sugars. Chloroplasts in plants are generally shaped like foot-balls.

Complexes of Pigments in Chloroplasts:

In early 1950s Robert Emerson noted that red light comprising wavelength exceeding 690nm was unproductive for photosynthesis despite fact that it was absorbed by chlorophyll. Though, when this light was supplemented by light having shorter wavelength photosynthesis happened faster than it did in either light alone. This effect become known as Emerson enhancement effect and it illustrated that plants have 2 light-harvesting systems. In all but most primitive bacteria, light is captured by network of chloroplast pigments arranged in aggregates on thylakoids. Such aggregates, known as antennae complexes are anchored in protein matrix and comprises of proteins approx 300 molecules of chlorophyll a, and about 50 molecules of carotenoids and other accessory pigments which collect light. Energy absorbed by antennae complexes flow energetically "downhill" to special pair of energy-collecting molecules of chlorophyll a and linked proteins called a reaction center. Though reaction centers comprise less than 1% of chlorophyll in plants, chlorophyll a in reaction center is electron acceptor which partakes directly in photosynthesis, all other photosynthetic pigments work as antennae. There are two types of reaction center chlorophylls, each containing light harvesters in their antennae. One of the chlorophylls absorbs maximally at 700nm and is called p700 (for pigment 700). Complex having p700nm and is known as photosystem I and comprises of eleven polypeptides, six of which are coded in nucleus, and five of which are coded in chloroplast. Three peripheral plypeptieds bind calcium and chlorine that describe why these nutrients are necessary for photosynthesis. Chlorophyll a molecule in photo system II is similar to that of photo system I, but it is related with different proteins, in photo system I, reaction center is bound to a large protein (Molecular weight 110,000). While that in photosystem II is bound to smaller protein (Molecular weight 47,000). Associated protein in photo system II shifts maximal absorption to about 680nm.

What occurs when Pigments absorb Light?

Light should be absorbed by pigment before it can have the effect. When light is absorbed, energy of its photons is captured by pigment and is utilized to boost energy of electrons, that is, it changes configuration of electrons by boosting then to higher energy orbitals. Such excited electrons can have many fates.

(a) Release of energy as heat.

(b) Isolated chlorophyll: electrons which had been raised fall to original energy level; light energy absorbed is released as heat and light (fluorescence).

(c) Intact chloroplast: energy passes to electron acceptors.

The energy can be released as heat (that is Molecular motion).

That energy can be released as afterglow of light via process known as fluorescence. Light released in fluorescence has longer wavelength (and thus less energy) than light which excited pigment. Chlorophyll fluoresces deep red. Isolated chlorophyll fluoresces as no molecules are nearby to accept energized electron.

Energy can be passed to neighboring molecule. Chlorophyll in thylakoids is enclosed by other molecules that trap energy of excited electrons.

Light driven reactions of photosynthesis that passes energy of photons to other molecules happen on photosynthetic membranes in plants and algae. These photosynthetic membranes are surrounded in chloroplast. In the organisms, chlorophyll is on membranes, in vesicles, or as in cyanobacteria in parallel stacks of flattened sacs.

Photophosphorylation-Chemiosmosis in Chloroplasts:

Photons absorbed by pigments energize electrons. Plants pass that energy by the series of molecules known as Electron Transport Chain. This transfer of electrons engages reduction and oxidation or redox, reactions. Electron donor is oxidized as electron acceptor is reduced. For electrons to flow down this chain every receiver should attract electrons more strongly than does donor. Potential energy of electrons drops at every step of electron transport chain plants couple this exergonic flow of electrons to the endergonic reaction which makes ATP. This light-driven production of ATP from electron transport is known as phosphorylation and relies upon the proton gradient. Photons captured by pigments excite electrons that are shuttled along carriers embedded in thylakoid membrane. When such electrons reach transmembrane H+ - pumps, the arrival induces transport of H+ across membrane, thus creating proton gradient which drives synthesis of ATP.

Chemiosmosis in chloroplasts (Photophossphorylation look like chemiosmosis in mitochondria (oxidative phosphorylation). Their parts of resemblance are given below:

1. Protons are pumped by the series of carriers which are gradually more electronegative.

2. Free energy released by electron transport produces and maintains the proton gradient across membrane. Though, there are some difference, for instance, in mitochondria energized electron moving by electron transport chain are extracted by oxidation for food in chloroplasts, light energizes electrons and no food is essential. In mitochondria, inner membrane pumps protons from matrix out to intermembrane space; this is reservoir of protons which powers synthesis of ATP. In chloroplasts, electron carriers in thylakoid pump protons from stroma in lumen (that is thylakoid space), that is proton reservoir. This light-driven pumping of protons into lumen decreases pH there to approx 5 while pH of stroma increases to 8 (that is, thousand fold difference in concentration of protons) light is needed to generate proton gradient; difference in pH across thylakoid membrane disappears rapidly in dark. Coupling factor proteins in the channels comprise ATP-synthase, the enzyme which harnesses flow of protons to make ATP. ATP produced by ATP synthase complexes is released into stroma, where it is utilized to make carbohydrates.

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